There are some OEM suppliers in China, but that probably won't help you.

The best place to start might be on a wheelchair racing forum, or by looking at some wheelchairs and seeing if you can find a rim with a decal on it.
It's also possible that a few aluminum makers offer them, but don't list with the bicycle products. Try your luck with Sun Metal, and maybe one of their people can give you a lead.

Over the years, I've found all sorts of good connections, by calling companies in relate areas, getting someone in sales or purchasing, and working leads from one to another until I found someone who could help. BTW- in most industries are small worlds, where everybody knows each other to some degree so once you have one name, it'll open doors to others, as in "I was speaking to XXX and he said you might be able to help".